There are many known types of optical recording elements including writeable optical recording elements. Writeable optical recording media can be recorded in real time. Information can be added to the CD at any time and can then be conveniently played back on a conventional CD player.
Writeable CD elements must meet a wide variety of requirements. First and foremost they must be readable using the large population of various CD players currently commercially available. This requires that parameters such as substrate material, substrate groove geometry, laser recording dye layer, recording layer conformality must all be optimized to work together as a parameter set. A change in one parameter in a set requires changes in the other parameters of that set.
Designers of optical recording elements have a variety of tools available to assist them in the fabrication of media which will conform to the various CD requirements. Foremost are the various computerized simulation techniques which are well known to those skilled in the art. The general description of the optics of laser readout systems and the computing procedures used for the simulations have been described by Hopkins, J. Opt. Soc. Am., Vol. 69, No. 1, p.4 (1979); G. Bouwhuis and J. Braat, Applied Optics and Opt. Eng., Vol. IX, (1983), p. 73; J. Braat, Proc. Int. Symp. on Optical Memory, Jap. J. of Appl. Phys., Vol. 28, (1989), Suppl. 28-3, p. 103; Y. Miyazaki and K. Manabe, Proc. Int. Symp. on Optical Memory, Jap. J. of Appl. Phys., Vol. 28, (1989), Suppl. 28-3, p. 109. The use of those tools are frequently cited in the literature, for example, by A. Inoue and E. Muramatsu, Proceedings of the ODS conference, Dana Point, Calif., 1994, Paper MA3-1 and U.S. Pat. No. 5,369,321 by Koike et al. These tools can be used to predict a desirable set of parameters when one or more of the parameters have been preselected.
Writeable CD elements are generally prepared by spin coating a dye solution on a grooved polycarbonate substrate. The dye forms the recording layer. Subsequent finishing is similar to that for pressed media; the dye layer is overcoated with a reflective layer and a protective layer. The parameter conformality (see FIG. 1) is determined in large part by the solvent used in the spin coating process. The solvent must apply the dye such that the amount of dye in the groove of the substrate and on the ungrooved portion (commonly called the `land` by those skilled in the art) is precisely controlled as described in U.S. Pat. No. 5,369,321 by T. Koike et. al. Only a very small number of solvents are known that will dissolve useful dyes and dye mixtures, without attacking the substrate.
Since the choice of suitable coating solvents is limited, and a given coating solvent results in a fixed conformality, the choice of other useful writeable CD parameters is also limited. For example, a common use of the above described computer modeling tools is to predict the substrate groove geometry required to accommodate a particular laser dye recording layer conformality. Limits in available conformality reduce the choice of substrate groove geometries.